1. Field of the Invention
The present invention relates to a fabric of thermoplastic fibers and continuous reinforcing fibers suited for being processed as a thermoplastic composite prepreg, with the thermoplastic fibers having a melting point lower than that of the reinforcing fibers.
2. Description of the Related Art
A description of how thermoplastic fibers and reinforcing fibers are processed to form a fabric is provided in EP-A-0182.335. The fabric is preferably heated under pressure to beyond the melting point of the thermoplastic material, so that the thermoplastic material will set around the reinforcing fibers and thus form the matrix of the composite product formed. The disadvantage of this prior art process is that during this so-called "thermoforming" of the prepreg, the thermoplastic fibers will shrink, while the absolutely non-shrinking reinforcing fibers, which, before the thermoforming, were substantially stretched and mainly parallel with the thermoplastic fibers in the fabric, have a surplus length and form "loops." Consequently, the mechanical properties of the resulting material are worse than they would be if the reinforcing fibers continued to be substantially stretched.
A process for the reduction of shrinkage problems in synthetic fiber is known from the clothing industry. Among the terms used for the process are annealing or heat-setting. Fibers made for use in clothing are subjected, if so desired, to a pre-shrinkage treatment using steam. Temperatures higher than that of steam are not applied, because, when used, clothing is generally not heated beyond such temperatures.
When the fibers are heated, they will shrink. In the processing of the fabric formed with these fibers, during which processing heat is supplied, the shrinkage is minimal, so that materials with favorable properties are produced.